Carbon fibers are used to form reinforcement textures in thermo-structural ceramic matrix composite (CMC) materials. The latter have properties that make them particularly apt to withstand large mechanical stresses at high temperatures. They are composed of a ceramic matrix deposited within the porous structure of a fibrous reinforcement, e.g. by chemical vapor deposition.
The manufacture of these materials is well known in the art. Specifically, a fibrous reinforcement made of carbon is used to construct a preform of a workpiece to be manufactured, either by winding fibers or yarns, or by piling one-dimensional-layers (plies of yarn or cable), or two-dimensional-layers (cloth or felt), possibly linked together by needling, or by three-dimensional weaving of fibers or yarns. The fibrous reinforcement, which may be held in shape by an appropriate tooling, is then densified by the matrix material. This densification can be achieved by liquid phase impregnation, using a matrix precursor, followed by a thermal treatment leaving behind a deposit of matrix material on the fibers within the reinforcement. Several impregnation cycles are generally necessary to obtain the required degree of densification. Densification can also be obtained by chemical vapor infiltration inside an infiltration furnace.
Many processes have been suggested for improving the behavior of such composite materials, especially their mechanical strength and oxidation resistance. In particular, a significant improvement in mechanical strength can be obtained by depositing, between the fibers of the reinforcement texture and the ceramic matrix, a think intermediate coating having a laminar structure, e.g. a layer of laminar pyrolytic carbon or boron nitride. Such a process is disclosed in document EP-A-0 172 082.
The present invention aims to provide a process yielding a substantial improvement in the properties of such composite materials, and in particular an increase in tensile strength and creep strength.